Defining delirium in idiopathic Parkinson's disease: A systematic review

•Delirium prevalence in Parkinson's ranged from 0.3 to 60% depending on setting.•Parkinson's was associated with an increased risk of developing delirium.•Delirium may be a risk factor for adverse outcomes in Parkinson's.•There was a paucity of well-designed, appropriately powered studies.